The present invention relates to an apparatus for winding and unwinding an elongated flexible member. More particularly, the invention relates to apparatus for winding and unwinding an elongated flexible member such as plastic film, sheet material or a linear member onto a winding rod with a specific tension exerted thereon. The apparatus of the invention may be readily applied to opening and closing a flexible screen in a greenhouse, or the like.
In general, when an elongated flexible member such as plastic film or sheet material is wound onto a winding rod rotating at a constant speed, the diameter of the roll increases in proportion to the length of the material that has been rolled up and, consequently, the winding speed of the elongated flexible member increases. When the elongated flexible member being wound is supplied at a constant speed, or is moved at a constant speed with the free end thereof supported, the tension exerted on said elongated flexible member increases in proportion to the length of the rolled up material. Consequently, the torque on the winding rod increases, resulting in the breakage of the elongated flexible member or in overload of the motor driving the winding rod. It is known that in such a case the tension exerted on the elongated flexible member may be maintained substantially constant by utilizing a torque motor whose rotational speed decreases in inverse proportion to the increase in the torque on the winding rod.
However, if the purpose is simply to wind the elongated flexible member onto the winding rod, this end may be attained by utilizing a torque motor and rotating the winding rod in only one direction. However, if it is necessary to unwind the elongated flexible member, difficulty arises. When opening or closing a flexible screen in a greenhouse, for example, even if the winding rod is rotated in the reverse direction by means of the torque motor, the flexible screen will not extend, but will accumulate near the winding rod. Therefore, it is necessary to pull the free end of the flexible screen, via another drive source, simultaneously with the reverse operation of the winding rod, with said flexible screen rolled up on said rod. However, since the diameter of the roll of flexible screen on the winding rod varies during this unwinding operation, if the winding rod with the flexible screen rolled up theron is reversed at a constant speed, the speed at which the flexible screen unwinds also varies. Accordingly, in order to extend the flexible screen uniformly and smoothly, it is necessary to vary the traction speed of the free end of the flexible screen in accordance with the variation in the roll diameter. Consequently, if the flexible screen is unwound from the winding rod with a constant traction speed, and if the speed of said winding rod in reverse is too high in the early stages of the unwinding operation, in which the diameter of the roll of flexible screen on said winding is large, said flexible screen is unwound excessively and accumulates. On the other hand, if the reverse speed of the winding rod is too low, an excessive tension is exerted on the flexible screen, resulting in breakage thereof.